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This page contains archived content and is no longer being updated. At the time of publication, it represented the best available science.

The Duck

On Sunday evening, December 19, the Terra flight
operations team was surprised when the satellites High Gain Antenna
spontaneously "safed itself." This antenna is the one that is used for
routine communications with TDRSS satellites, including the downlink of
Terras science data. The flight operations team quickly initiated a series
of diagnostic tests to find out what went wrong.

Quinn explains that there are built-in electronics in the High Gain Antenna
that constantly monitor the electric current being drawn by the motor drive
assembly, which controls the pointing direction of the antenna. If the electric
current either exceeds or falls below a certain limit, then the antenna is
preprogrammed to flag the problem and can decide to park itself until the flight
operations team decides to restart it. In this case, telemetry data from the
spacecraft indicated an anomalously high current passed through the motor drive
assembly.

"At first, only one axis on the antennas gimbal was affected and
the other axis kept moving," Quinn recalls. "Then an additional fault
detection shut down the other axis. We knew right away that this wasnt due
to a failure of the High Gain Antenna. So we began a brainstorming session in
which we listed all the possible reasons why this could have happened."

"It turns out, there is a semi-conducting opto-coupler in the High Gain
Antenna electronics that is susceptible to radiation transient single event
upsets," Grady explains. Thats a technical way of saying that the
satellite was exposed to a high dose of proton radiation in the Earths
magnetic field. Because electronic devices are getting smaller and smaller, they
are increasingly susceptible to interference or damage from radiation.

Terra's high gain antennathe parabolic dish shown here above the spacecrafttransmits science data
from the five instruments to scientists on the ground through a relay satellite. Without it, data from Terra would slow
to a trickle. (Image and video by Reto Stöckli, Terra Project Science Office [4.7MB])

There are three regions over the Earth where scientists typically observe high
levels of radiationover the North and South Poles, and another region centered
partly over Brazil and extending out over the Atlantic Ocean that scientists call
the "South Atlantic Anomaly." In this region, scientists observe very
high levels of proton radiation. The Terra flight operations team nicknamed the
region "The Duck" because when you draw a contour map of the central
part of the South Atlantic Anomaly where the radiation levels are highest, it
resembles the profile of a duck.

According to Grady, exposure to radiation in The Duck essentially fooled
Terras High Gain Antenna into thinking that a fault condition existed in
its Motor Drive Assembly. The antenna responded by turning itself off in order
to prevent any possible damage from occurring.

Interestingly, the Multi-angle Imaging SpectroRadiometer, or MISR, instrument
aboard Terra confirmed that the spacecraft was being exposed to high levels of
radiation over the South Atlantic Anomaly. By February, the instrument had
already been turned on and, although its aperture doors were still closed, MISR
was fully ready for operation. Every time Terra flew through The Duck, MISR
measured very high numbers of protons striking its highly sensitive detectors.
Intrigued, the MISR Team began counting these "hits" and produced the
first MISR image before they even opened its doors!

The region of the South Atlantic Anomaly was dubbed "The Duck" by the Terra flight operations team because when you draw a contour map of the central part of the anomaly, it resembles the profile of a duck. (Image by Mark Woodard, NASA Goddard Space Flight Center)

Even before the cover opened, the Multi-angle Imaging SpectroRadiometer
(MISR) instrument aboard NASA's Terra spacecraft began making scientific
measurements. The
MISR cameras, designed to detect visible light, are also sensitive to energetic
protons in Earth's upper atmosphere. With the cover closed, background levels of
protons stand out.
This map was created by specially processing MISR "dark" data taken between February 316, 2000, while the cover was still closed. Each orange picture
element on the map shows one or more proton hits. (Image courtesy MISR Science team)

Of course, Gradys team spent a few days brainstorming and running
diagnostic tests to be sure the South Atlantic Anomaly was the cause of the
problem. Once convinced, and after making sure there was no danger to the
antenna, they simply modified the computer program to restart the antenna gimbal
subsequent to a radiation event. Quinn says the antenna still turns off over
this region about once every 30 orbits, but then turns itself on again with no
impact to mission operations.

"There was quite a bit of effort expended to make sure we didnt
have these kinds of parts [overly sensitive to radiation anomalies] in the
electronics," Grady states. "Of the tens of thousands of parts that we
examined before launch, this one somehow got by us. This is where a robust
spacecraft design is critical, as it allows the flight controllers to implement
an operational modification with no resulting impact on the missions
science. The antenna continues to work fine."

With the antenna problem solved, Gradys team began preparations for
raising Terra to its final orbital altitude for mission activation. They
didnt know it yet, but the satellite had another surprise in store for
them.